This invention relates to methods of transmitting data over a communications link and to cell-based methods of transmitting data over a communications link and to corresponding systems, apparatus and software for transmitting or receiving data and in particular to transmitting such data over a communications link which is subject to a high error rate, such as a wireless link.
Cell-based protocols for transmission systems are well known. Cells (also called packets) are sent individually, and routed over a network using addressing information in the cell. Usually the data needs to be fitted into a number of such cells. Some protocols specify variable length cells, for example IP, and others specify cells of fixed length, such as ATM. Some are connection oriented, such as SNA, ATM, X.25 and frame relay. Others are connectionless, for example IP.
One known cell-based protocol, the Asynchronous Transfer Mode (ATM) protocol, will now be discussed in more detail. The ATM protocol is designed for data transfer over high speed, low error rate digital networks for multiple service types. ATM has generally been considered unsuitable for wireless transmission due to its low tolerance to errors.
ATM Adaptation Layers (AALs) shape the service data for the ATM protocol and provide error protection characteristics dictated by the properties of the transmission medium and the service. There are currently a number of different AALs specified, each with different error handling features. For example AAL 5 includes error detection information, though not correction information, at the frame level, but none is provided at the cell level, at least for the cell payload. This error protection is achieved by concatenating each data unit with a header and trailer, resulting in a variable length frame structure. The frame is then segmented into cells for transmission by the ATM protocol.
ATM is a transmission protocol based upon asynchronous time division multiplexing using fixed length data packets or cells. These cells typically have a length of 53 bytes: each cell containing 48 octets of user data (payload) and 5 octets of network information (header). Cells of a length of 55 bytes are also known, but in less frequent use. The ATM protocol segments data into cells. The header of a cell contains limited header error correction information. The header of a cell also contains address information which allows the network to route the cell. The address information is made from a concatenation of the Virtual Path Identifier and the Virtual Channel Identifier. These two fields require 28 bits (3.5 octets) of information. FIG. 1 shows an example of an ATM cell header.
ATM communications technology will have an important part to play in the evolution of global communications networks, especially the internet. Considering trunk communication, each trunk link will be used to carry several different types of traffic, the two most common traffic types being generally known as voice and data. Voice traffic can contain errors and still be understood to a reasonable quality due to the brain""s ability to cope with noise. The key criterion of voice is that it cannot withstand large variations in delay. Data traffic, such as a file transfer, can tolerate extreme delays but cannot tolerate lost or corrupt information. In general these two different types of traffic have been carried by different networks operating significanty different protocols. More recently Asynchronous Transfer Mode (ATM) has provided a common network protocol for these two traffic types.
One characteristic of ATM is the provision of an idle cell. Some data sources exhibit a variable bit rate and in order to support the transmission data from such sources it is sometimes necessary, when the bit rate decreases, to transmit empty cells, referred to as idle cells. This enables ATM to transport traffic with a wide range of characteristics.
ATM has been designed to operate over low error-rate trunk networks, which generally use reliable optical communication techniques, and assumes that the data traffic suffers a low error rate. The international Telecommunications Union (ITU) has recommended that erroneous frames be recovered by retransmissions of the entire frame in preference to individual cell retransmissions. Assured services carry significant disadvantages because of the retransmission system, including prolonged buffering at the transmitter, prolonged buffering at the receiver, complex protocol acknowledgement structures, and increased latency from acknowledgement messages and data retransmission.
Wireless communication is becoming one of the most popular commercial methods for providing access and trunk communications. With the recent launch of Low Earth Orbit (LEO) satellites, there are now many applications within commercial and military environments that may use terrestrial or satellite-based wireless communication links. The problem with wireless communication is that the fundamental error rates are significantly greater than those experienced in wireline systems. Wireless systems must cope with a harsher signal propagation environment that is subject to noise, interference, fading and delay. This is further compounded by the restrictions on the power levels at which wireless systems can operate. Mobile handsets and orbiting satellites are restricted in their transmit power by battery life, and wireless systems are generally constrained by regulatory limits on transmit power.
Wireless communication suffers errors within the traffic and the distribution of errors is uneven. The error rate has an underlying random independent bit error rate, overlaid by a burst error rate. Whilst ATM has therefore generally been considered unsuitable for wireless transmission, some attempts have been made to adapt ATM for transmission over wireless systems.
U.S. Pat. No. 5,568,482 (Li et al., assigned to Yurie Systems Inc.) describes a low speed radio link system for ATM transport. An incoming stream of ATM cells intended for transmission over the radio link is segmented into a plurality of subframes, each subframe carrying a plurality of ATM cells and having additional framing bytes. One example uses nine subframes, each carrying five ATM cells. The structure of this protocol allows synchronisation to be more easily maintained under burst error conditions on the link.
U.S. Pat. Ser. No. 09/222,557 (Bentall, assigned to Nortel Networks Corporation) describes a technique for improving ATM operation over a communications link whereby bandwidth is improved on a communications link by sending a header that supports fewer addresses. An 8 bit address field permits 256 different addresses. This header is associated at both ends of the link such that the original header can be removed, the packet is associated with one of the 256 available channels, and the original header is reconstructed at the far end. By doing this some bandwidth is gained which can be used to improve the quality of the link.
Radio channels such as those found in access networks and radio links typically exhibit bit-error rates (BER) of 10xe2x88x923 to 10xe2x88x925. These error rates are too high for ATM transmission.
The solutions available in the open literature typically rely on a combination of forward error correction (FEC) and backward error correction (BEC or ARQ). This results in excessive cell delay variation. Furthermore, they often allow an increase in transmitted bit-rate, which restricts the choice of switch/radio combination, as the link-enhancing device is no longer transparent in this sense. It is believed that this occurs in several link accelerators for satellite radio links. However, as ATM has been proposed for the next generation of military communications, some form of link hardening is seen as essential in military radio links for both terrestrial and satellite applications.
The present invention seeks to provide a more reliable method for transmitting data packets over a communications link such as a wireless link. The present invention further seeks to provide a means of improving the BER of a radio link so that it is low enough to meet the requirements of the ATM protocol and in particular the Cell Loss Ratio (CLR) and the Cell Error Ratio (CER) without the introduction of excessive Cell Delay Variation (CDV)
The present invention further seeks to provide a radio link protocol that affords robust communications between a headend and a plurality of mobile or fixed substations in a standardised data communications format, such as the ATM data format. The present invention also seeks to provide a radio link protocol for transferring ATM cells that has improved framing information to maintain link synchronisation under high error and burst error conditions.
According to a first aspect of the invention, there is provided a method of providing error correction for a cell based transmission protocol; wherein the cells are arranged to form multi-cell frames; wherein the header field of the multi-cell frame comprises all the headers of the cells within the multi cell frame and wherein the payload field of the multi-cell frame comprises all the payloads of the cells within the multi-cell frame; wherein the header field of the multi-cell frame and payload field of the multi-cell frame are coded separately, and; wherein idle cells within the payload of the multi-cell frame provide space for error correction of the payload field.
Preferably the header field of each cell is reduced in size. The header field can be used to optimise payload coding. For a plurality of multi-cell frames, the idle cells can be evenly distributed across said plurality of multi-cell frames. The cell-based protocol can be the ATM protocol.
For a plurality of multi-cell frames, the number of idle cells can be allocated according to the steps;
i) determining the number (N) of multi-cell frames;
ii) determining the number (N) of idle cells within said N multi-cell frames, and:
iii) allocating the first N idle cells to the N multi-cell frames such that each frame contains an idle cell,
The link can be a satellite link or a terrestrial link Preferably, for a plurality of multi-cell frames, the idle cells are evenly distributed across said plurality of multi-cell frames and wherein buffers are employed to evenly distribute the idle cells. The buffers can comprise a preceding buffer and an active buffer to evenly distribute the idle cells. The number of traffic cells to output in each frame is governed by a set of rules depending on the number of traffic cells in both the active and preceding buffers. The number of traffic cells to output in each frame is governed by a set of rules depending on the number of traffic cells in both the active and preceding buffers, which rules are set to limit the maximum cell delay variation possible to that set in the cell based protocol standards.
Preferably, the number of traffic cells to output in each frame is governed by a set of rules depending on the number of traffic cells in both the active and preceding buffers, which rules are set to limit the maximum cell delay variation possible to that set in the cell based protocol standards according to the following table:
Preferably, an algorithm is provided which is operable to evenly distribute these idle cells amongst the frames. The separate coding of the headers and payloads can enable at least some payload errors to be removed by reference to the header field. Where idle cells are present in the frame, the presence of an idle cell headers in the header field of the frame is used to enhance the effectiveness of the coding applied to the payload field.
Previously attempts have been made to eliminate the presence of idle cells in order to allow more bandwidth to be allocated for error correction coding. Difficulties arise in the provision of coding for a channel having a variable bandwidth. Further, the transmission of idle cells has been viewed as an ineffective use of bandwidth, although the primary use of idle cells is to allow the transmission protocol flexibility in varying rates of data flow.
In accordance with a second aspect of the invention, there is provided a method of transmitting data in accordance with a cell-based protocol, the method comprising the following steps:
arranging the data cells into multi-cell frames;
reducing the header size;
separately coding the cell headers and payloads;
providing a synchronisation word;
wherein the coding of the headers and payloads is selected from a pre-determined coding-set according to traffic load and communications channel conditions.
The invention accordingly provides a method of improving the resilience of the cell-based protocol to radio link errors. The invention can also improve the performance of a radio link so that it can support the cell-based protocol. This is achieved by combining cell-based protocol cells into a frame to which is added forward error correction (FEC) coding and a synchronisation word. The method exploits the variability in the cell-based protocol traffic rate and redundancy in the cell-based protocol itself to provide bandwidth for the FEC coding so that no increase in overall bit rate is required.
The present invention thus avoids the use of backward error correction (BEC) techniques such as automatic repeat request (ARQ)xe2x80x94and without expansion of the transmitted bit-rate. For example, in ATM, the avoidance of the use of ARQ is essential if the ATM standard for cell delay variation (CDV) is to be met. Maintaining a constant bit-rate is essential if the link-enhancing device, incorporating the invention, is to be inserted transparently between the switch and the radio.
A synchronisation word is added to the frame to provide frame synchronisation. After reception by the receiving radio, which is assumed to achieve bit synchronisation; frame synchronisation is achieved using the synchronisation word. The synchronisation algorithm is readily derived from information contained in the open literature and is based on examination of several (typically six) synchronisation words simultaneously. The use of a small synchronisation word in this way improves the performance in high error rate environments.
The algorithm re-distributes idle cells to optimise the coding strength. This enables the system to adapt to traffic load. In order to smooth the variation in rate at which traffic cells arrive, a procedure controlled by the algorithm is provided whereby the efficiency of the FEC coding is improved. An example of such a mechanism would have a buffer, referred hereafter as the active buffer, containing 3 frames (each frame consisting of 5 cell-based protocol traffic cells). Another buffer preceding the active buffer would contain the next frame in the sequence to those in the active buffer, this frame is used to give an indication of the incoming traffic rate. From the number of traffic and idle cells present in the active buffer, and the number in the preceding buffer, cells can be chosen to make up an output frame with the optimum distribution of traffic and idle cells for the FEC requirements and the traffic loading present. The number of traffic cells to output in each frame is governed by a set of rules depending on the number of traffic cells in both the active and preceding buffers. These rules are set to limit the maximum cell delay variation possible to that set in the cell based protocol standards. An example of these rules is
In summary, the invention provides a method wherein the header fields of the cells are compressed in order to incorporate error correction coding for the headers and makes use of idle cells to provide space for error correction for the payloads of the cells, an algorithm is provided which seeks to evenly distribute these idle cells amongst the frames and the separate coding of the headers and payloads enables some payload errors to be removed by reference to the header field, thus increasing the effectiveness of the payload coding. The system is appropriate for cell-based protocols such as the ATM protocol.
In accordance with a further aspect of the invention, there is provided a system for transmitting data across a link in accordance with a cell based protocol and comprising a link enhancer, the link enhancer being positioned for transmission between a cell based protocol switch and a channel modulator, and, for reception, between a channel modulator a cell-based protocol switch, the link enhancer being operable to:
i) arrange cells into multi-cell frames;
ii) arrange the header field of each multi-cell frame such that it comprises all the headers of the cells within the multi-cell frame and arrange the payload field of each multi-cell frame such that it comprises all the payloads of the cells within the multi-cell frame;
iii) code separately the header field of the multi-cell frame and payload field of the multi-cell frame, and;
iv) provide space for error correction in idle cells, whereby to optimise code performance using the idle cells.
In accordance with another aspect of the invention, there is provided an apparatus for transmitting data across a link in accordance with a call based protocol and comprising a link enhancer, the link enhancer being positioned for transmission between a cell based protocol switch and a channel modulator; and, for reception, between a channel modulator a cell-based protocol switch, the link enhancer being operable to:
i) arrange cells into multi-cell frames;
ii) arrange the header field of each multi-cell frame such that it comprises all the headers of the cells within the multi-cell frame and arrange the payload field of each multi-cell frame such that it comprises all the payloads of the cells within the multi-cell frame;
iii) code separately the header field of the multi-cell frame and payload field of the multi-cell frame, and;
iv) provide space for error correction in idle cells, whereby to optimise code performance using the idle cells.
In accordance with a still further aspect of the invention, there is provided an apparatus for a receiver for receiving across a link in accordance with a cell based protocol and comprising a link enhancer, the link enhancer being positioned for transmission between a cell based protocol switch and a channel modulator; and, for reception, between a channel modulator a cell-based protocol switch, the link enhancer being operable to:
i) receive multi-cell frames;
ii) arrange the header field of each multi-cell frame such that it comprises all the headers of the cells within the multi-cell frame and arrange the payload field of each multi-cell frame such that it comprises all the payloads of the cells within the multi-cell frame,
iii) decode separately the header field of the multi-cell frame and payload field of the multi-cell frame, and;
iv) analyse the received data for errors using error correction data encoded within idle cells, and;
v) convert the signals to a cell based protocol, the error correction coding allowing reduced errors whereby to optimise code performance using the idle cells.
For a plurality of multi-cell frames, the number of idle cells can be averaged. The cell-based protocol can be the ATM protocol. Preferably the number of traffic calls to output in each frame is governed by a set of rules depending on the number of traffic cells in both the active and preceding buffers, which rules are set to limit the maximum cell delay variation possible to that set in the cell based protocol standards.